The Noam Elimelech runs with this idea and explains something fantastic:
When we perform a mitzwah, we move or gesture in a certain way. This particular motion that we do is a revellation of our Godly soul, a revellation of HaShem's infinite light in this finite world. When somebody else sees that motion and imitates it, they have none of the same intentions or spiritual forces at work, all they are doing is moving their body. In other words they are performing a meaningless action for its own sake. This, the Noam Elimelech explains, is avodah zarah. They didn't even understand that what they found desirable about the gesture was not the gesture itself but the soul revealed in that particular gesture. This is why the gestures of Tzaddikim cause people great joy.
This is a kind of abstract idea, but it has cool implications. I think it pays to look at it like this:
When someone smiles at you, they are expressing a spiritual joy at seeing you. When this mkaes you smile, your soul is sharing and reciprocating that joy. This makes all the difference, because when someone smiles at you just to perform the act of smiling at you, it doesn't make you feel the same way, there's no soul in that gesture, just the gesture itself.
In a sense smiling without meaning it is what the Noam Elimelech is talking about, it is a lie that does nothing but increase the uncertainty in the world, it is a kind of Avodah Zara. [A subtle point: if you are smiling to be nice out of derech eretz, then that is something else entirely.]
The Baal Shem Tov explained this experience: We can see that it is the soul within a person that interests us, because if the person were lying lifeless before you, the body is still there, but you wouldn't address them with the same interest.